Electronic key systems for accessing restricted areas are known and take a number of forms. These include magnetic card readers, keypad door locks, interrogator/transponder systems and various transmitter/receiver type systems.
Magnetic card readers and keypad door locks offer minimal security or convenience improvements compared to conventional mechanical locks. The user must physically contact the card reader or keypad to obtain entry. Both of these access control systems could easily be compromised by the loss of the magnetic card or inadvertent disclosure of the keypad code. In either case an unauthorised person could access the restricted area.
Interrogator/transponder systems and transmitter/receiver systems are more sophisticated in that they do not require any physical action on the part of the authorised person. These systems permit access based on the proximity of a key or token to an access controller.
One early example of the interrogator/transponder security system is described in U.S. Pat. No. 3,891,980 assigned to Lewis Security Systems Ltd. This patent describes a system in which an authorised person carries a token that generates signals at two discrete frequencies when the person approaches a door leading to a restricted area. A sensor detects the person's presence and activates a control unit which interrogates inductive loops adjacent the door to determine if the correct signals are being picked up. If the correct signals are present the door is opened automatically, otherwise the door remains locked and an alarm may be given.
The prior art includes many examples of transmitter/receiver security systems designed primarily for automatic locking and unlocking of automobile doors. Early devices required the user to push a button on a transmitter that sent a radio frequency signal to a receiver to activate solenoid-operated electronic door locks. More recent devices have removed the necessity for any physical action by the user and work entirely on the basis of proximity.
One such device is described in U.S. Pat. No. 4,688,036 assigned to Nissan Motor KK. This patent describes a portable code signal transmitter that is approximately the size and shape of a credit card. The transmitter produces a radio frequency signal with a unique code that is detected by a controller in a vehicle. Vehicle functions are actuated by the controller when the unique code matches a preset code stored in the controller.
Another device is described in U.S. Pat. No. 4,942,393 assigned to Lectron Products Inc. This patent describes a system to automatically unlock and lock the doors of an automotive vehicle as a user approaches or moves away from the vehicle. The system consists of a portable beacon carried by the user, a receiver/controller in the vehicle and an antenna connected to the receiver controller. The patent describes a number of improvements over earlier devices including a motion sensor to conserve battery life and transmitter signals embodying differential phase encoded data with error correction coding of the data to enhance noise immunity and signal discrimination.
The prior art patents have also addressed the problem of a user leaving the transmitter in the vehicle, one example being U.S. Pat. No. 4,672,375. In this patent the controller has many of the features described above but also includes a disabler feature. The disabler becomes active when the vehicle door is closed and locked, and disables the entry system if the unique code is detected for a period exceeding a predetermined time. An alarm sounds to remind the user to collect the transmitter.
The prior art sometimes refers to these systems as `keyless` referring to the absence of conventional key and lock mechanisms although in the strictest sense the mechanical key has merely been replaced by an electronic key. The unique code transmitted by the electronic key is analogous to the teeth pattern of a mechanical key. The electronic key can be more versatile and the transmitted signal can provide functional information as well as an identifying code. The ability to provide an alarm if the electronic key is left in the vehicle is also a valuable advantage.
Although having a number of advantages over conventional keys all the known prior art devices suffer from the same problem of an unauthorised person being able to access a restricted area (including a motor vehicle) by simply gaining access to the signal generating device. Thus a device that is lost, left or stolen can negate all of the security advantages embodied in the "keyless" entry system.
This shortcoming is highlighted where such a system is embodied as a motor vehicle security system. It is common for the signal generating device to be embodied as a tag that is attached to a key ring holding the vehicle keys. A carjacker can readily access a vehicle and operate the ignition if they have the key ring with the tag attached. Prior art devices that embody the signal generating device in the key head only exacerbate the problem.
Clearly, an arrangement is needed which is disabled when separated from the authorised person thereby preventing unauthorised persons from gaining access to and use of the restricted area even if they hold a signal generating device.
Devices to achieve this object exist in the form of retina pattern and fingerprint recognition systems. While such devices would overcome the stated problem they are too expensive and insufficiently portable to be a practical solution to the problem of vehicle and facility security (except in larger facilities). A more economically and practically viable solution is required.